OSMOTIC PRESSURE AND THE DETERMINA- 

 TION OF MOLECULAR WEIGHTS 1 



WITHIN recent years it has become more and more appar- 

 ent that an intimate connection exists between the stoichio- 

 metrical composition of a solution and its physical behavior. 

 While earlier efforts towards proving this assumption were 

 unsuccessful, chiefly because the experimental material was 

 almost wholly confined to aqueous solutions of salts, which 

 offer peculiar obstacles to such an investigation, F. Raoult's 2 

 patient researches upon organic substances in more varied 

 solutions have enabled him to formulate accurately the long- 

 expected laws, and to put the chemical world under lasting 

 obligation for new methods for determining molecular 

 weights under very favorable conditions. Raoult's law would 

 read, in its most general form : In dilute solutions the depres- 

 sions of the vapor tension and of the freezing-point of the 

 solvent vary directly with the ratio between the numbers of 

 molecules of solvent and of substance dissolved in the mix- 

 ture. Provided there be no chemical action involved in the 

 process of solution, this relation is entirely independent of 

 the nature of the substances. 



Within the ordinary range of conditions, the freezing-point 

 of a pure liquid is constant; according to Raoult's law, every 

 molecule of foreign matter occasions the same constant de- 

 pression. Every liquid, however, has its own constant coeffi- 

 cient of depression, which may be found by determining the 

 depression caused by the presence of one molecule of any 

 substance in one hundred molecules of the liquid. 



1 A brief review of preceding work published in the American Chemical Journal, 

 12, 130 (1890). 



2 Campt. rend. 87, 167 (1878); 95, 1030 (1882). 



